1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling a soft buffer size for a transport block and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3rd Generation Partnership Project (3GPP) Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3GPP as a successor of the universal mobile telecommunication system (UMTS) for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and for communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint (CoMP) transmissions/reception, uplink (UL) multiple-input multiple-output (UL-MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
A rate-matching procedure targets to match the number of bits in a transport block (TB) to the number of bits that can be transmitted within a transmission time interval (TTI) when a transmitter (e.g., eNB) prepares to transmit/retransmit the TB to a receiver (e.g., UE), wherein the number of bits transmitted within the TTI may be determined according to a soft buffer size for the TB for the rate-matching procedure. A hybrid automatic repeat request (HARQ) process starts to operate, when a receiver (eNB or UE) cannot decode a received packet successfully. The receiver stores soft values of a transmitted/retransmitted TB in a soft buffer of the receiver, and combines the soft values to increase a probability of successful decoding, wherein a size of the soft values stored by the receiver may be determined according to a soft buffer size for the TB for storing the soft values. The receiver continues the HARQ process until the TB is decoded correctly, or until a maximum number of retransmissions have been sent, at which time the HARQ process declares a failure and leaves it up to the HARQ process in radio link control (RLC) layer for trying again.
In a legacy LTE system, e.g., the LTE system or the LTE-A system, a maximum modulation level of 64 quadrature amplitude modulation (64QAM) is supported, and the storage efficiency of the soft buffer may not be a big issue for the receiver. However, an increased maximum modulation level (e.g., 256QAM) may be supported, when an enhanced technology (e.g., small cell enhancement) is implemented to improve performance of the legacy LTE system. That is, the maximum number of bits of a downlink shared channel (DL-SCH) TB received within a TTI may also be increased. In this situation, a method designed for partitioning a soft buffer of a UE and/or a network in the legacy LTE system may not be efficient for the increased maximum modulation level. Not only a blocking probability of a HARQ process may be increased in this scenario, but also coding/retransmission performance of the HARQ process may be degraded. The problem may be even worse when a carrier aggregation (CA) is supported, where multiple cells (e.g., multiple component carriers) are operated at the same time and a larger number of HARQ processes may occur.
Thus, improving the performance of the HARQ process is an important problem to be solved.